1. Field of the Invention
The present invention relates to a magnetic disk apparatus for magnetically storing and reproducing information, and more particularly, to a magnetic head having a magnetoresistive head for reproducing information by using giant magnetoresistance effect and a magnetic disk apparatus using such a magnetic head.
2. Related Background Art
There are known two types of magnetic heads for magnetically reproducing information by using magnetoresistive (MR) effect. One is a so-called AMR head, which is a type using anisotropic magnetoresistive (AMR) effect in which a micro magnetoresistive change is caused by an angle between the direction of magnetization of a single magnetic film and the direction of a current, and the other is a GMR head, a type using giant magnetoresistive (GMR) effect obtained by a change in an angle between the magnetizing directions of at least two magnetic layers. For example, U.S. Pat. No. 4,755,897 discloses a spin valve structured head for reading signals by fixing the magnetizing direction of one of two magnetic layers (fixed magnetizing direction layer) and by freely turning the other magnetizing direction (rotatable magnetizing direction layer). The spin valve structure can be applied with little change in the structure and manufacturing process of the magnetoresistive head developed as the AMR type. Although the spin valve structured head has an advantage that sensitivity is increased two to four times greater than that of the AMR head, there is a problem such that application of a magnetic field in a magnetizing process is different from that in the AMR head.
A sensor part according to the spin valve structure has a hard bias structure such that a rotatable magnetizing direction layer, for example, NiFe alloy, is formed on a proper underlayer. A non-magnetic interlayer, a magnetoresistive change auxiliary layer, a fixed magnetizing direction layer, and an antiferromagnetic layer are sequentially formed. Further, a magnetic domain control layer for limiting an MR sensor layer only within the sensor part and an electrode are formed. With respect to a bias relation of the spin valve structure, it is necessary to change the magnetizing direction of the rotatable magnetizing direction layer so as to be perpendicular to the height direction of the MR sensor by means of a magnetic field obtained by synthesizing a current bias field Hi, a magnetostatic coupling field Hbt from the end of the fixed magnetizing direction layer, an interlayer coupling field He, and a bias field Hb1 from the magnetic domain control layer by applying a sensing current. Consequently, the magnetizing direction of the rotatable magnetizing direction layer has to be in parallel to the height direction of the MR sensor. A recording magnetic field invades the height direction of the MR sensor and vertically turns the magnetizing direction of the rotatable magnetizing direction layer. In this instance, a signal in which the resistance is minimum when an angle .theta. with the fixed magnetizing direction layer is 0 and the resistance is maximum when the angle .theta. is 180.degree. C. is obtained. Consequently, the magnetizing direction of the rotatable magnetizing direction layer has to be directed to the track width direction.
When the surface roughness of the interface is increased, the interlayer coupling is enhanced and aerial distribution is increased. It is necessary to set the thickness of each spin valve structured magnetic layer to be equal to or less than 10 nm and the surface roughness to be 1 nm or less. However, a lower shielding layer as a base on which the sensor part is formed has the thickness of 1 to 2 .mu.m. When crystalline alloys such as NiFe and FeAlSi are applied to the lower shielding layer, the crystal grain size on the surface of the shielding layer is 0.5 to 1 .mu.m. When grain orientations are different, growth rates become different. A wave of 5 to 10 nm in the surface roughness Rrms, which is equal to or larger than each film thickness, is caused. Therefore, in order to use those alloys, it is necessary to set the surface roughness of the magnetic under layer to 1 nm or less by flattening the shielding layer by the etch back or by polishing the lower gap film with sacrifice of the accuracy of the film thickness. On the other hand, when an amorphous film wherein the surface roughness becomes 1 nm or less is used as the lower shielding layer, there are no problems caused by the roughness. In the AMR head, there is no inconvenience when the amorphous layer is used as the lower shielding layer.
However, if the amorphous shielding layer is applied to the GMR head, when heat treatment for magnetization of the fixed magnetizing direction layer is performed there can occur instances wherein the magnetic domain structure deteriorates according to the temperature of the heat treatment, and the time and the direction of the applied field, so that the waveform fluctuates. The waveform fluctuation is characterized in that asymmetry is lost after operating the write head and returns to its former state after recording operations of a few times to hundreds of times. This might constitute the reason that the magnetic domains appear in the shielding layer which are located at a distance of 0.1 to 0.2 .mu.m from the sensor part and are coupled magnetostatically to the magnetization of the magnetic domain control layer and lose the controllability of the magnetic domain, or the magnetostatic coupling of the shielding layer and the magnetoresistive film is changed, thereby changing bias. The following reason is considered as a cause of the appearance of the magnetic domains in the shielding layer. A fabricating step of the magnetoresistive head consists of a wafer step and a post step. The wafer step comprises: a step (step 1) of forming a lower shielding layer on an insulating layer applied on a substrate; a step (step 2) of forming a lower gap layer; a step (step 3) of forming a spin valve structured layer, an electrode, and a magnetic domain control layer; a step (step 4) of forming an upper gap layer, and after that, forming an upper shielding layer; a step (step 5) of forming a write head gap layer, an inductive coil, an interlayer insulating film, and an upper magnetic core; and a step (step 6) of forming a protective layer, a terminal, and the like. In the post step, a process for the air bearing surface, and cutting, adhering using a gimbal, and the like are performed.
In the post step, the heat treatment by applying the magnetic field cannot be given since a slider chip is not easy to handle, it is feared that a slider is deformed, and the like. In the wafer step, the steps in which the heat treatment can be given are limited according to the magnetization temperature of the fixed magnetizing direction layer. Specifically, when NiMn that is required to be subjected to the heat treatment at 250.degree. C. or higher is used for fixing the magnetization of the magnetizing direction layer, since the heat resisting temperature of the write head interlayer insulating film is 230.degree. C., the heat treatment has to be given by the end of the step 4. In this case, there is a problem that the magnetization of the shielding layer oriented in the height direction of the MR sensor is not returned by the following heat treatment in the track width direction. On the other hand, since NiO has to be cooled from 200.degree. C. in the magnetic field, it has to be done in the step 6. In this case, the final heat treatment in the height direction of the MR sensor is given to the lower shielding layer and the magnetization is disturbed.